IFixit, which recently expanded its own offerings to include a Wikipedia-like site for repairs, says "the immediate visible difference is the inclusion of a black plastic RF window on top of the iPad for better antenna reception."

The new iPad boasts five antennas in all, two for cell reception, two for Wi-Fi/Bluetooth connectivity and one more for GPS. In fact, iFixit says Apple is using the whole LCD frame as an antenna.

Other insights from iFixit iare that Apple used the same baseband processor from Infineon for both iPad models, but that the iPad 3G has a Broadcom BCM4750UBG Single-Chip AGPS, whereas the iPhone 3GS uses an Infineon Hammerhead II. IFixit even details what kind of screws are used to hold the iPad together.

IFixit says a "dramatic shift" with the iPad 3G is that it is not tied to a specific carrier, even though in reality for U.S. customers AT&T is the only provider whose 3G network uses the frequencies supported by the device. Apple has taken much grief for locking customers into AT&T with its iPhone over the years, though recently has become more flexible (at a price).

Under the covers, the iPad 3G has more pieces than the original iPad, but it appears there is plenty of room to add new pieces as demand for new capabilities arise.

The 3G model costs from $629 to $829, and runs $130 more than corresponding models of the Wi-Fi-only version. There are two 3G service options: $15 per month for a 250MB metered plan and $30 a month for an unlimited data offering, as is the case with the Apple iPhone 3G.

The iPad has gotten off to a fast start, at least according to Apple, which delayed sales overseas because it said popularity in the U.S. strained supplies. What's more, a typically sure sign that something has hit it big: malware purveyors have already begun trying to scam iPad enthusiasts into downloading their malicious code. In addition, rumors are swirling about what direction HP will take its tablet plans in, perhaps in reaction to the iPad's early success, and Microsoft has confirmed that its Courier tablet has been shelved.

Bob Brown is a news editor for Network World, blogs about network research, and works most closely with our staff's wireless/mobile reporters. Follow him on Twitter at Alphadoggs and connect via email at bbrown@nww.com